Search for: All records

ABSTRACT Tomographic images hold key constraints for North American Cordillera plate-tectonic histories, but models that prioritize surface geology (i.e., top-down) or mantle tomographic slabs (i.e., bottom-up) show disparate conclusions. Here, we highlight five key questions regarding assumptions and uncertainties for building North American Cordillera plate reconstructions from tomography: (1) Which seismic anomalies under North America should be interpreted as subducted slabs? (2) How should paleo–subduction zone absolute locations be determined from mantle constraints? (3) How should paleo–subduction zone polarities be determined from tomography? (4) How should slab constraints be integrated into absolute plate motion models? (5) How do inferences from lower-mantle slabs compare to surface geologic remnants of accreted terranes along western North America? The lower mantle under North America shows five main slab-like, faster-than-average seismic anomalies; of these, four anomalies are generally interpreted as subducted slabs, and the other anomaly is possibly a geophysical artifact. Comparison of interpreted subducted slabs from published tomography shows the interpreted slabs generally occupy similar mantle locations, but slab dips may be highly variable between tomographic data sets. Thus, caution is needed when inferring paleo–subduction zone polarity or absolute locations directly from slab dips. Absolute plate motion models of areas offshore western North America during the Jurassic–Cretaceous imply a relatively fast, northeast-moving Farallon oceanic plate that precludes a stationary, long-lived (>40 m.y.), west-dipping intra-oceanic subduction zone. However, long-lived east-dipping or bivergent (east- and west-dipping) intra-oceanic subduction zones are permissible. Alternative conclusions about North American Cordillera paleo–subduction zone polarities and terrane-slab correlations stem from slab reconstruction approaches based on “vertical slab sinking” and “nonvertical slab sinking” hypotheses; future tests of these hypotheses will improve our understanding of Earth’s mantle and its convective processes. 

Abstract In Tao et al. (2025,https://doi.org/10.1029/2024GL110777), we presented a high‐resolution seismic anisotropy model, proposing that the Cape Verde (CV) hotspot caused enhanced radial anisotropy in the North American lithosphere. We also suggested that this interaction could have created zones of low‐lying topography that set the stage for later formation of the Great Lakes by glacial scouring. Peace et al. (2025,https://doi.org/10.1029/2025GL115634) agreed that glacial scouring created the Great Lakes, but they disputed the contribution of the CV hotspot. They also questioned aspects of our interpretation of the anisotropy and the uniqueness of hotspot tracks across North America. Here we address these points and further explain the consistency of the Tao et al. (2025,https://doi.org/10.1029/2024GL110777) anisotropy model with the track of the CV hotspot across eastern North America. 

Abstract Shear wave splitting (SWS) patterns at subduction zones are often interpreted by complex mantle flow above or below the slab. However, our recent previous work shows dipping anisotropic slabs can explain observed patterns in Japan. Here, we extend this analysis to the Alaska subduction zone, using 2,567 high‐quality teleseismic SWS measurements from 195 broadband stations. As was found in Japan, the observed SWS patterns in Alaska depend on earthquake backazimuth. The fast‐S polarization directions are either trench parallel or perpendicular in southeastern Alaska and form a prominent circular pattern in central Alaska. We found that a dipping anisotropic slab following the Slab 2.0 geometry, with 30% shear anisotropy, and exhibiting tilted transverse isotropy with a symmetry axis normal to the slab interface, predicts both the fast‐S polarizations and delay times (δt = 1.0–1.5 s). This suggests that intra‐slab anisotropy can be the primary control on SWS, without requiring complex mantle flow. 

Abstract Interaction between Tethys and the Paleo‐Pacific subduction zones in Southeast Asia during the Mesozoic remains poorly understood. Using new and published zircon U‐Pb and Hf data sets from Borneo (Paleo‐Pacific domain) and Sumatra (Tethyan domain), we propose that isotopically juvenile magmatism was active on both sides of Sundaland due to the initiation of inward‐dipping double subduction during the latest Triassic when Indochina collided with Sibumasu, as evidenced by a pronounced positive shift in zircon εHf(t) values from both Cenozoic sedimentary successions and Mesozoic magmatic rocks in Sumatra and Borneo. From the latest Triassic to Cretaceous, the contrasting positive εHf(t) values ranges between Borneo and Sumatra, with Borneo showing a broad range and Sumatra a narrower variability, imply that the inward‐dipping double subduction system evolved asymmetrically due to differences in slab dip angles between the subducting Meso‐Tethys and Paleo‐Pacific oceanic lithosphere. After 80 Ma, this asymmetric double subduction system was disrupted, marked by the complete cessation of arc magmatism in Borneo while isotopically juvenile magmatism continued on Sumatra. Our findings emphasize that, when compared to the contemporary single‐sided subduction system of the western Meso‐Tethyan domain and the northern Paleo‐Pacific domain, SE Asia developed more juvenile crust due to large‐scale upper plate extension driven by inward‐dipping double subduction. 

Abstract Detecting old hotspot tracks in a stable continent remains challenging because of the lack of volcano chains on the surface and the fade of thermal anomalies with time. The northeastern American continent moved over the Cape Verde and the Great Meteor hotspots during 300–100 Ma. However, only the latter was confirmed by kimberlites and seismic velocity models. Our new 3D anisotropic model in northeastern America reveals strong positive radial anisotropy anomalies in the eastern Great Lakes, central Pennsylvania, and northwestern Virginia. These anomalies follow the Cape Verde hotspot track, providing the first geophysical evidence for the hotspot. A circular pattern of azimuthal anisotropy is also observed in the eastern Great Lakes and may be related to the Cape Verde plume activity. The plume was under the Great Lakes during 300–200 Ma and probably caused lithosphere thinning and low topography needed for forming the Lakes during the glacial era. 

Abstract The slab thermal state model predicts that only cold slabs should retain some of their intra‐slab water beyond subduction zones, while warmer slabs should be nearly dry past the volcanic arc front. Such predictions are yet to be fully tested, as they mostly rely on numerical modeling. To further test the slab thermal model, here we have examined slab‐sensitive elemental and isotopic tracers in recently erupted basalts (<5 Myr) from along and across an arc transect at end‐member types of cold (NE Japan) and hot subduction zones (SW Japan and Ryukyu) and beyond (eastern China intraplate volcanism). We show that the oceanic crust and the incoming hydrated mantle from the cold subducted Pacific plate are the main water carriers beyond subduction zones. Only cold slabs may thus recycle part of their intra‐slab H₂O into the lower mantle. Warmer slabs are too dry past the back‐arc or too short‐lived to exert a first order control on deep water recycling. 

Abstract Delineation of geochemically distinct domains in Earth’s mantle is essential for understanding large-scale mantle convective flow and dynamics. Previous studies identify possible long-lived (>60 million-year) mantle isotopic domains (i.e. Antarctic-Zealandia, Pacific and Indian) near the Philippine Sea and western Pacific. Here we compile published basalt geochemistry of the Philippine Sea and surroundings and add new Mo isotopic and water content data for Gagua Ridge lavas, northwestern Philippine Sea, to distinguish slab-derived components during subduction. The water content, trace element, and Mo-Sr-Nd isotope compositions of Gagua Ridge arc lavas suggest that slab fluids and sediment melts are responsible for element recycling to the arc. The Philippine Sea basalts show both Indian and Zealandia-Antarctic Pb isotopic signatures; restoration of the basalt locations within a plate reconstruction shows the far-travelled Philippine Sea traversed these mantle domains. We establish the Indian mantle domain eastern boundary at ~120°E under SE Asia and the Indian Ocean. The Antarctic-Zealandia mantle domain lies south of ~10°N within the SW Pacific and has mostly remained in oceanic realms since ~400 Ma with only limited continental material input. 

Abstract The Alaska Peninsula has a long history of plate subduction with along‐arc variations in volcanic eruption styles and geochemistry. However, the sub‐arc melting processes that feed these volcanoes are unclear. The Alaska slab morphology below 200 km depth remains debated due to limited seismic data and thus low tomography resolution in this region. Here we utilize the newly available regional and teleseismic data to build 3‐D high‐resolutionV_PandV_Smodels to 660 km depth. We find that the high‐velocity Pacific Plate subducts to the bottom of the mantle transition zone (MTZ) with complex deformation and gaps. In the southwest, we observe a wide gap in the high‐velocity slab at 200–500 km depths. Toward the northeast, the slab becomes more continuous extending to the MTZ with a few holes below 200 km. We interpret these gaps as a slab tear that coincides with the subducted ancient Kula‐Pacific Ridge. We also invert for 3‐DV_PandV_P/V_Smodels to 200 km depth with higher resolution and find strong along‐strike changes in slab dehydration and sub‐arc melting, indicated by lowV_Pand highV_P/V_Sanomalies. Slab dehydration and sub‐arc melting are most extensive below the Pavlof and Shumagin segments in the southwest, becoming limited below the Chignik and Chirikof segments in the northeast, and extensive again beneath the Kodiak segment further to the northeast. We propose that the variations of slab hydration at the outer rise significantly influence slab dehydration at greater depths and further control sub‐arc melting beneath the Alaska Peninsula. 

Abstract The Mesozoic subduction history of the Paleo‐Pacific plate below the East Asian margin remains contentious, in part because the southern part is poorly understood. To address this, we conducted a sediment provenance study to constrain Mesozoic subduction history below West Sarawak, Borneo. A combination of detrital zircon U‐Pb geochronology, heavy minerals, trace element, and bulk rock Nd isotope data were used to identify the tectonic events. The overall maturity of mineral assemblages, dominantly felsic sources, abundant Precambrian‐aged zircons, and low εNd(0) values (average −13.07) seen in Late Triassic sedimentary rocks suggest a period of inactive subduction near Borneo. Slab shallowing subduction occurred between 200 and 170 Ma based on subdued magmatism and tectonic compression across West Sarawak. From c. 170 to 70 Ma there was widespread magmatism and we interpret the Paleo‐Pacific slab steepened. Collectively, we show the Paleo‐Pacific plate subduction had variable slab dip histories in Borneo. 

SUMMARY Differences between P- and S-wave models have been frequently used as evidence for the presence of large-scale compositional heterogeneity in the Earth's mantle. Our two-step machine learning (ML) analysis of 28 P- and S-wave global tomographic models reveals that, on a global scale, such differences are for the most part not intrinsic and could be reduced by changing the models in their respective null spaces. In other words, P- and S-wave images of mantle structure are not necessarily distinct from each other. Thus, a purely thermal explanation for large-scale seismic structure is sufficient at present; significant mantle compositional heterogeneities do not need to be invoked. We analyse 28 widely used tomographic models based on various theoretical approximations ranging from ray theory (e.g. UU-P07 and MIT-P08), Born scattering (e.g. DETOX) and full-waveform techniques (e.g. CSEM and GLAD). We apply Varimax principal component analysis to reduce tomography model dimensionality by 83 percent, while preserving relevant information (94 percent of the original variance), followed by hierarchical clustering (HC) analysis using Ward's method to quantitatively categorize all models into hierarchical groups based on similarities. We found two main tomography model clusters: Cluster 1, which we called ‘Pure P wave’, is composed of six P-wave models that only use longitudinal body wave phases (e.g. P, PP and Pdiff); and Cluster 2, which we called ‘Mixed’, includes both P- and S-wave models. P-wave models in the ‘Mixed’ cluster use inversion methods that include inputs from other geophysical and geological data sources, and this causes them to be more similar to S-wave models than Pure P-wave models without significant loss of fitness to P-wave data. Given that inclusion of new data classes and seismic phases in more recent tomographic models significantly changes imaged seismic structure, our ML assessment of global tomography model similarity may improve selection of appropriate P- and S-wave models for future global tomography comparative studies.